This invention relates to the production of anhydrides of aromatic naphthenic and saturated aliphatic carboxylic acids. More specifically, this invention relates to the production of anhydrides of polycarboxylic acids having at least one pair of carboxyl groups attached to adjacent carbon atoms of the molecule. The anhydride portion of the molecule therefore has the configuration ##SPC1##
As compared with the structure of other anhydrides which may be generally represented as: ##SPC2##
Which are generally formed either by condensation of two molecules having the general formula R--COOH, (in which R may be hydrogen, or alkyl, cycloalkyl, aryl, heterocyclic, naphthenic, etc., with or without other substituents) or from a polycarboxylic acid having carboxyl groups attached to non-adjacent carbon atoms, e.g., terephthalic acid.
The predominant process for the production of anhydrides having at least one pair of carboxyl groups attached to adjacent carbon atoms is by air oxidation of a corresponding hydrocarbon or hydrocarbon derivative, such as an aldehyde, in the presence of a catalyst. For example, anhydrides of aromatic carboxylic acids such as phthalic, trimellitic and pyromellitic acids are commonly prepared by air oxidation of corresponding lower alkyl (e.g., methyl-, ethyl- and/or propyl-) substituted benzenes. In the case of phthalic anhydride the hydrocarbon is usually either naphthalene or ortho-xylene; trimellitic acid is commercially prepared by air oxidation of pseudocumene and pyromellitic acid from durene. Similarly, anhydrides of aliphatic carboxylic acids of this type may also be prepared from corresponding hydrocarbons or derivatives thereof.
A second method of preparation of such anhydrides is by dehydration of the corresponding carboxylic acids.
Such processes are known to produce significant amounts of oxidized by-products, such as aldehydes, other carboxylic acids, ketones, etc. In some cases, the presence of such by-products may not be greatly disadvantageous to the process, because the anhydride may not be required in great purity. However, in some cases the presence of significant amounts of by-products will derogate from the suitability of the anhydride for the particular end use. For example, in the preparation of polyesters or polyamides from the anhydrides, the presence of impurities in significant amounts can interfere with the formation of appropriate polymer chains or result in cross-linking. Additionally, such by-products may give the polymers an undesirable color and/or other undesirable properties. For example, phthalic anhydride, when produced by the oxidation of orthoxylene, will contain small but significant amounts of partly oxidized compounds such as ortho-toluic acid and phthalide and over-oxidized compounds such as maleic anhydride and benzoic acid. Removal of such impurities is an expensive process, even though they may be present in quite small amounts, often less than 1 percent.
Purification of phthalic anhydride generally requires heating to polymerize some of the impurities and several distillation steps to separate and recover the anhydride.
More recently it has been found that aromatic carboxylic acids can be prepared by conversion of the corresponding hydrocarbons to nitriles, followed by conversion of the nitriles to acids, e.g., by hydrolysis. The hydrolysis may be performed with or without the aid of a catalyst, and with stripping of the ammonia from the hydrolysis product, and results in the formation of ammonium salts of the acids. Anhydrides of acids having at least one pair of carboxyl groups attached to adjacent carbon atoms, e.g., phthalic, trimellitic and pyromellitic can be obtained by thermal decomposition of the ammonium salt(s). However, such processes also tend to produce substantial amounts of the corresponding imide, and subsequent purification of the anhydride will be required to eliminate this and other nitrogen-containing by-products such as intermediate hydrolysis products, which could interfere with the production of polyesters or polyamides.
The carboxylic acids and/or their ammonium salts may also be produced by hydrolysis of corresponding amides or imides.
It is an object of the present invention to produce anhydrides of aromatic, naphthenic and saturated aliphatic carboxylic acids without requiring production of the acid as an intermediate compound. More specifically, it is an object of the present invention to produce anhydrides of aromatic, naphthenic and saturated aliphatic polycarboxylic acids having at least one pair of carboxyl groups attached to adjacent carbon atoms. A second object of this invention is to produce anhydrides of such carboxylic acids free from the presence of undesirable oxidation by-products. Another object of the present invention is to provide anhydrides of such carboxylic acids suitable for use in the production of polyesters, polyamides, and other polymeric compounds. Yet another object of the present invention is the production of anhydrides of such aromatic and aliphatic carboxylic acids from the corresponding nitriles. A further object of the present invention is to produce such anhydrides from amine salts of the corresponding acids. A still further object of the present invention is to produce such anhydrides from the nitriles of the corresponding acids, with the production of the amine salt of the acid as an intermediate, and without requiring separation of the amine salts from the hydrolysis products.